Read the Passage carefully and answer the questions.
The carbonyl group of aldehydes and ketones is highly polar. The positively charged carbon is readily attacked by electron-rich nucleophile and the negatively charged oxygen atom is attacked by an electro-deficient electrophile. Aldehydes and ketones undergo nucleophilic addition reactions in the presence of acid or a base. The mechanism involves attack of a nucleophile on the electron deficient carbonyl carbon to form a tetrahedral intermediate. A new bond is formed between the carbon and oxygen. Protonation of the anion furnished by the reaction medium yields a neutral addition product. The addition product is same whether the reaction is catalyzed by acid or a base. For example in the presence of base aldehydes and ketones react with hydrogen cyanide to form cyanohydrin.
Which of the following reactions is not a nucleophilic addition reaction of aldehydes and ketones?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Addition of sodium borohydride **
The reaction which is not classified as a nucleophilic addition reaction of aldehydes and ketones is: Addition of sodium borohydride
BECAUSE
- NaBH₄ causes reduction of aldehydes and ketones to alcohols.
- Although hydride (H⁻) attacks the carbonyl carbon, this reaction is classified as a reduction, not as a nucleophilic addition reaction in standard organic chemistry classification.
Nucleophilic addition reactions include:
- Addition of water → hydrate formation
- Addition of sodium hydrogen sulphite (NaHSO₃) → bisulphite addition compound
- Addition of alcohol → hemiacetal/acetal formation